Perhaps Anastasia has a special power within her that is linked to the AllSpark. In the fanfiction, the Transformers could be drawn to her because of this power. The story could explore how she copes with suddenly being in the middle of an intergalactic struggle between the Autobots and Decepticons.
One idea could be that Anastasia is a long - lost Cybertronian in human form. The Autobots discover her true identity while on Earth and have to protect her from the Decepticons who want to use her unique connection to both Earth and Cybertron for their evil plans.
The Autobots, like Optimus Prime, would be noble and protective towards Anastasia. They would see her as an important part of their mission on Earth. Optimus might be a mentor - like figure to her, guiding her in the new world she has entered. The Decepticons, on the other hand, would view her as a means to an end. Megatron might try to manipulate her into helping him get what he wants.
In a Transformers and Bumblebee crossover fanfiction, Bumblebee could lose his memory and the other Transformers have to help him regain it while also protecting him from the Decepticons who see this as an opportunity to take him out. They go on a journey through different planets and encounter various strange species along the way.
Perhaps there is a portal that connects Cybertron and Equestria. Some ponies accidentally end up on Cybertron and are shocked by the mechanical world. The Transformers have to help them get back home. Meanwhile, some Autobots are curious about the ponies' world and decide to accompany them. This leads to a cultural exchange of sorts between the two groups, with lots of misunderstandings at first but then growing respect and friendship.
There could be a situation where a Transformer's technology gets mixed with Oscorp's experiments. This creates a new kind of threat that Spider - Man has to face. The Transformers get involved because it's their technology that's been misused. Together, they figure out a way to reverse the effects and defeat the new hybrid - like villains.
One plot idea could be that the Star Trek crew discovers a planet where Transformers have been in a long - standing war. The Enterprise crew tries to broker peace between the Autobots and Decepticons. Another could be a situation where the technology of the Transformers somehow gets integrated into the Star Trek universe's ships, creating super - powered vessels. Or perhaps a Transformer gets beamed onto the Enterprise and has to adapt to life among the crew while they figure out how to send it back to its own world.
One exciting plot idea could be that the Fast and Furious crew discovers a hidden Transformer technology while on a heist. This technology gives their cars new, almost super - power - like abilities. For example, their cars could transform into smaller, more agile forms for tight getaways. Another idea is that the Transformers need the help of the Fast and Furious team to retrieve an important artifact. The humans' driving skills are crucial in navigating through dangerous terrains filled with Decepticons. And a third idea could be a race where both humans and Transformers participate. The Transformers have to adjust their massive forms to fit the rules of the race, and the Fast and Furious crew has to use all their tricks to keep up with the alien machines.
One idea could be that the Jaegers are equipped with Transformer technology. For example, their power cores could be replaced with energon, giving them new abilities. Another plot could involve the Transformers teaming up with the Jaeger pilots to fight a common enemy that threatens both Earth and Cybertron. Maybe a new breed of Kaiju that can also assimilate Transformer technology.
A great plot could be that a powerful artifact from the Transformers universe is found on Earth and it has the power to either save or destroy the planet. The Avengers initially think it's a new weapon or power source, while the Autobots know its true origin. They have to convince the Avengers of its importance and together protect it from the Decepticons and any other villains who want to use it for their own evil purposes.
One possibility is that the Angry Birds are mistaken for magical creatures in the Anastasia world. Anastasia, believing they can help her find her true identity, enlists their aid. The Angry Birds then have to deal with the political intrigue and danger in the Anastasia story while also trying to find a way back to their own world. For example, they might have to avoid the clutches of the villainous Rasputin (if we consider him part of the Anastasia story in this crossover) who could see the birds as a threat to his plans.
One idea could be to focus on the interaction between the Autobots and the characters from 'The Last Knight' universe. For example, how would Optimus Prime react to the ancient secrets and magic in that world? Another inspiration could be the concept of transformation in both. The Transformers can transform physically, while in 'The Last Knight' there might be metaphorical or magical 'transformations' of characters. You could explore how these different forms of transformation intersect.